This invention relates to a center support grinding method, a center support grinding machine, and a centering method for centers thereof in which a cylindrical workpiece to be subjected to outer diameter machining is held by the two centers and fed to a grinding wheel while being rotated, in particular, to a center support grinding method, a center support grinding machine, and a centering method for centers thereof which are suitable for the grinding of the peripheral surface of a cylindrical workpiece with a small diameter and in which it is easy to miniaturize the grinding machine for grinding per se.
In the grinding of the cylindrical surface of a cylindrical workpiece of minute size, for example, in the grinding of the cylindrical surface of a Zr ferrule for an optical connector, an optical fiber insertion hole of 0.125 mm is formed at the center of a cylinder with an outer diameter of 2.5 mm to 1.25 mm concentrically with the outer diameter, and a concentricity on the order of submicrons is required between the insertion hole and the outer diameter.
In today""s world, where the markets of automobiles and household electrical appliances have reached saturation and where the demand for computers and information equipment has increased, the technical field is expanding where there is a requirement for precision grinding of the cylindrical surface of a cylindrical workpiece of minute size constituting a mechanical part (a rotation shaft in a hard disk apparatus, a recording head rotation shaft in a video camera, a bearing therefor, etc.) for use in such products as are in increasing demand.
Incidentally, a conventional grinding machine has been used for precision grinding of such a cylindrical-surface of minute size, the grinding machine having on a base of great mass and high rigidity heavy and robust tables for moving a workpiece and a grinding wheel, there being provided on these tables a heavy and robust workpiece retaining spindle and a grinding wheel retaining spindle. Usually, the main body of this conventional grinding machines for minute workpieces has a floor area of 1 m2 and a weight of close to 1 ton. In an example, a workpiece having a diameter of 4 mm, a length of 10 mm, and a weight of 1 g is machined by a machine one million times as heavy as that.
On the other hand, in the case of grinding the cylindrical surface of a general mechanical part, for example, a workpiece having a diameter of 4 cm, a length of 10 mm, and a weight of 1 kg, it is machined by a machine tool with a floor area and a weight of not more than 10 m2 and 10 tons, which means the ratio of the weight of the machine to the weight of the workpiece is approximately 10 thousand.
Thus, the grinding machine for machining a workpiece of minute size occupies and exhibits a large floor area and a large weight which are out of proportion to the workpiece. This excessively large grinding machine is based on the idea of xe2x80x9cThe larger serves for the smallerxe2x80x9d. That is, the grinding machine for machining a workpiece of minute size is endowed with the ability to machine a relatively large workpiece, and the grinding wheel driving motor is large and heavy and exhibits an accordingly large output. The grinding wheel base on which the large and heavy driving motor is placed is inevitably large and heavy. Further, the table on which the workpiece and the grinding wheel are to be placed is also larged and heavy. Further, the feed screw for moving these heavy tables is thick, and the driving motor for the feed screw is large and heavy.
It is to be assumed that this tendency of the grinding machine for machining a workpiece of minute size to be excessively large and heavy is attributable to the following conventional circumstances:
(1) No machine tool dedicated to minute parts has been commercially produced.
(2) In purchasing a machine tool, it is generally believed that the larger the size and capacity, the better.
However, in machining a minute size workpiece, such as a ferrule, the rotation shaft of a hard disk apparatus, the recording head rotation shaft of a video camera, and the bearings thereof, the volume of the portion removed by machining is small, and the requisite power for machining is also small.
Thus, for the machining of a minute size workpiece, running a large and heavy machine tool by a high power motor, constructing a large building of high load capacity for installing the large and heavy machine tool, and providing a wide air conditioning facilities for accommodating the machine tool, are superfluous and wasteful.
By using a motor of an output, weight, and size suitable for the machining of a minute size workpiece and appropriately reducing the size and weight of the spindle stock, table, etc. it is possible to machine a minute size workpiece without involving an excessively large machine tool, excessive energy consumption, or excessive plant facilities.
After studying this possibility, the present inventor has found out that it is possible to reduce the size and weight of a machine tool so as to realize a machine which is approximately 20 to 30 kg in weight and 20 to 30 cm across in size and which can be raised and moved by hand.
If such a miniaturized machine is realized, it would provide the following advantages from the economical viewpoint. It is possible to reduce the requisite power for the machine tool itself. It is also possible to reduce the price of the machine, the plant facility cost, and the plant running cost, such as the air conditioning cost. Further, when the machine is out of order, instead of depending on the conventional in-field services, which involve a high cost and a long downtime, it is possible to obtain a substitute from the maker by using courier service, thereby recovering the failure in a short time and at low cost.
Specifically speaking, in realizing a reduction in the weight and size of a grinding machine for machining a minute part, the following are to be taken into account: supply and discharge of a minute workpiece, rotary drive, feed, in-process sizing, etc.
In cylindrically grinding a cylindrical workpiece, a chuck-drive/center-support system is widely used, in which the forward end of a workpiece chuck gripped by a main shaft chuck is center-supported. Further, in a known lathe using the chuck-drive/center-support system, the centers are rotated in synchronism with the chuck to eliminate relative rotation between the workpiece and the centers to thereby achieve an improvement in rotation accuracy (See, for example, patent document 1).
Patent Document 1
JP 2000-71104 A (See Paragraphs 0019 and 0020, and FIGS. 1 and 2)
In the chuck-drive/center-support system, however, the chuck has a rather large outer size and requires much space, with the result that the arrangement space for the workpiece supply/discharge device, the rotary drive device, the feed device, the in-process sizing device, etc. is rather small. Further, grinding is performed on an outer configuration basis, and not on a center-hole basis.
Generally speaking, to machine a cylindrical workpiece on a center-hole basis with high concentricity, the optimum method to be adopted is a two-center support type machining system, in which the cylindrical workpiece is held, with the forward ends of a pair of centers being inserted into center holes provided in the end surfaces of the cylindrical workpiece. However, in machining a small diameter cylindrical workpiece, such as a Zr ferrule, it is necessary to arrange a machining tool such as a grinding wheel, a workpiece supply/discharge device, a sizing device, etc. close to each other in a small space around the workpiece, which results in a poor operability if ordinary xe2x80x9ccarriet turningxe2x80x9d is adopted, thereby hindering a reduction in size and weight.
Instead of xe2x80x9ccarriet turningxe2x80x9d, patent document 2 discloses a ferrule rotating method using a rubber roller as shown in FIG. 11. In FIG. 11, a ferrule 1 constituting a cylindrical workpiece is elastically supported between a stationary center 101 and a tailstock center 102 axially movable but not rotatable by the resilient force of a pressurizing spring 103, and the cylindrical workpiece 1 is presses by a rotating rubber roller 104 from the direction opposite to a rotating grinding wheel 20 to rotate the cylindrical workpiece 1 by frictional force. In order that a sufficient frictional force may be obtained between the contact surfaces of the cylindrical workpiece 1 and the rubber roller 104, the cylindrical workpiece 1 is held in press contact with the rubber roller 104 with a force strong enough to form a recess in the rubber roller 104.
Patent Document 2
JP 10-113852 A (Japanese Patent No. 3171434) (paragraphs 0017 through 0019, FIG. 2)
In this ferrule rotating method, there is no need to change the clamping position and perform grinding two times as in the case of the xe2x80x9ccarriet turningxe2x80x9d, in which the cylindrical surface to be ground is clamped. Thus, the method is superior in operational efficiency and provides an improved concentricity for the cylindrical workpiece 1.
When the workpiece 1 held by the two centers is rotated, the forward ends of the centers 101 and 102 and the center holes of the workpiece 1 slip on each other. Since the cylindrical workpiece 1 is pressurized in opposite directions by the rubber roller 104 and the grinding wheel 20, equilibrium in force can be achieved during grinding in-feed. However, in the condition before and after actual grinding, in which the grinding wheel 20 is not in contact with the cylindrical workpiece 1, and in the finish grinding step, the cylindrical workpiece 1 is pressurized in one radial direction by the rubber roller 104. The period of time in which the cylindrical workpiece 1 is rotated before and after actual grinding and the period of time of the finish grinding step are longer than the period of time for grinding in-feed, and, all the while, the centers 101 and 102 are pressurized in one radial direction by the center holes of the cylindrical workpiece 1. Thus, as a large number of cylindrical workpieces 1 are repeatedly ground, xe2x80x9cpartial wearxe2x80x9d tends to be caused by frictional force. The smaller the diameter of the cylindrical workpiece and the center hole diameter, the more conspicuous becomes this xe2x80x9cpartial wearxe2x80x9d. It is to be assumed that this is attributable to a reduction in the contact area between the centers and the center holes.
Furthermore, usually, this xe2x80x9cpartial wearxe2x80x9d of the centers is not uniform between the two centers 101 and 102. In particular, when the hole diameters of the centers on the right and left sides of the cylindrical workpiece 1 are different, the partial wear is always nonuniform. Although not so serious as in the case of xe2x80x9ccarriet turningxe2x80x9d, this nonuniformity in xe2x80x9cpartial wearxe2x80x9d on the right and left sides leads to a certain degree of defective cylindricality of the ground cylindrical surface of the cylindrical workpiece 1.
To avoid this defective cylindricality, the cylindricality of the cylindrical workpiece 1 after grinding is monitored, and when the permissible range has been approached, or when a fixed number of workpieces have been ground, the grinding machine is stopped, and fine adjustment is empirically performed on the center positions, or the centers are replaced for positional adjustment.
In a case where a high degree of precision in cylindricality is required, the frequency of center adjustment and replacement increases even with this rotating method, with the result that the availability factor is reduced, and the center consumption increases, which constitutes an obstruction to a reduction in production cost.
Further, nowadays, there is an increasing demand for a high precision machining enabling a cylindrical workpiece with a very small outer diameter of approximately 1.25 mm to be machined with a high degree of cylindricality. However, when the outer diameter of the cylindrical workpiece 1 is diminished, the rotation by the rubber roller 104 becomes difficult.
Further, the presence of the rubber roller does not contribute to a reduction in size; it diminishes the space around the workpiece to some degree, and somewhat reduces the degree of freedom in the arrangement of the sizing device, the supply/discharge device, etc.
The present invention has bee made in order to solve the above-mentioned problems, and an object thereof is to provide a grinding machine and a centering method for the centers thereof which is suitable for center-hole-referenced high precision grinding of a workpiece with a small diameter, in which the requisite space for the center support rotating mechanism for a workpiece and the in-feed mechanism is reduced to facilitate miniaturization, and in which it is easy to secure the space for the supply and discharge of the workpiece and sizing measurement.
In order to attain the above-mentioned object, a center support grinding method according to the present invention is characterized in that a cylindrical workpiece is supported by two centers, and that the workpiece is ground while being rotated by the two centers.
Further, a center support grinding method of the present invention is characterized by including: a rough grinding step for performing rough grinding on a cylindrical workpiece while rotating the workpiece by the two centers holding the workpiece; and a finish grinding step for performing, after the rough grinding step, finish grinding on the workpiece while rotating one center, with the other center being fixed in position.
In those methods, the following structure may be adopted in which the two centers are rotated in synchronism with each other by separate built-in motors, or the two centers are rotated by separate built-in motors in order that the fixation of one center is effected by a stationary constraining force of the built-in motor.
Further, a center support grinding machine according to the present invention is characterized by including: a main shaft unit; a main spindle rotatably retained in the main shaft unit; a rotary drive center retained by the main spindle and adapted to be engaged with one center hole of a cylindrical workpiece; a main spindle rotary drive means built into the main shaft unit and adapted to rotate the main spindle; a tailstock unit; a tailstock spindle retained in the tailstock unit so as to be slidable in the axial direction; a tailstock center retained by the tailstock spindle, arranged so as to be opposed to the rotary drive center in the same axis, and adapted to be engaged with the other center hole of the cylindrical workpiece to hold the cylindrical workpiece together with the rotary drive sensor; a tailstock center urging means for elastically urging the tailstock center toward the rotary drive center side to hold the cylindrical workpiece between the rotary drive center and the tailstock center; and an in-feed means on which the rotary drive center, the main spindle rotary drive means, and the tailstock center are mounted and which moves the rotary drive center, the main spindle rotary drive means, and the tailstock center by a swiveling motion to thereby cause the cylindrical workpiece held between the centers and rotated to make an in-feed operation with respect to a grinding wheel.
Further, in the above-mentioned center support grinding machine, the tailstock spindle is rotatably retained by the tailstock unit, and there is further provided a tailstock spindle rotary drive means built into the tailstock unit and adapted to rotate the tailstock spindle in the same direction as the main spindle. Furthermore, the main spindle rotary drive means and the tailstock spindle rotary drive means are rotated in synchronism with each other. Otherwise, at least one of the main spindle rotary drive means and the tailstock spindle rotary drive means has a stationary constraining force.
Further, the tailstock spindle rotary drive means is an inner rotor type electric motor, an inner rotor of the motor being attached to the tailstock spindle, and an outer stator thereof being fixed to the tailstock unit so that the inner rotor of the motor moves in the axial direction with respect to the stator when the tailstock spindle moves in the axial direction.
Further, in the above-mentioned center support grinding machine, the following may be adopted in which the tailstock center urging means also serves as a tailstock spindle axial movement means for moving the tailstock spindle in the axial direction, or the tailstock center urging means is provided in the tailstock unit, and the tailstock unit is movable in the axial direction of the tailstock spindle, moving in the axial direction of the tailstock spindle by the spindle axial movement means.
Further, the following may be adopted in which the in-feed means retains the rotary drive center, the main spindle rotary drive means, and the tailstock center by an eccentric bearing eccentrically arranged with respect to the rotary drive center and the tailstock center, and is adapted to make an in-feed operation through swiveling of the eccentric bearing, or that that the in-feed means has an in-feed lever on which the rotary drive center, the main spindle rotary drive means, and the tailstock center are mounted, the in-feed operation being made through swiveling of the in-feed lever.
A centering method for centers of a rotary drive center device according to the present invention is a method in which the centers are respectively mounted to opposing center mounting holes of a pair of spindles arranged in the same axis and in which a cylindrical workpiece is rotated while being held between the centers, characterized in that the centers are respectively mounted to the spindles and that the centers are ground for centering by a grinding tool while rotating the spindles.
In the above-mentioned method of the present invention, the following may be adopted in which a single grinding tool is equipped with grinding surfaces for grinding a pair of opposing centers, the centers being ground simultaneously for centering by the grinding tool.